Why is the US even $1 in debt if, as the Constitution mandates, Congress creates the nation's legal tender?  After all, any sovereign nation will normally create its own currency, spend it into circulation, and through taxation or other means remove some currency from circulation for value regulation.
So why does the US government borrow at all?
Answer: Because neither Congress, nor any government branch or agency or public bank, creates anything other than the nation's coinage (i.e. pennies, nickels, dimes, quarters, etc).  In other words the US does NOT have a sovereign currency.  It's all created, controlled, owned, and distributed by PRIVATE banks.  Even if we ignore the fact that the Federal Reserve itself is private, we still can't get past the fact that its member banks are private and that's where all digital currency (the vast majority of all currency) comes from via their loan process.
So where does money come from?  Read here... 
BECAUSE Congress doesn't create the nation's currency, it must get the money from somewhere else in order to have anything to spend on what the government is supposed to spend money on (like the military).  The only sources: Taxes, borrowing (bond sales), and/or selling government products & services.
So, the One Key to the national debt, the root cause of it ultimately, is that all currency (whether printed or digital) other than coins and a few remaining US Notes is created by PRIVATE BANKS as debt, first by the private (private shareholder owned) Federal Reserve, and via fractional reserve lending by the Fed's private member banks (the Fed's shareholders).  It doesn't belong to the US, it belongs to the Federal Reserve and represents a pile of IOU's. 
We have to understand that there's a difference between GOVERNMENT debt, and PUBLIC debt, and that GOVERNMENT debt is a result of BORROWING from the PUBLIC debt thus creating a 2nd layer of debt.
It's important also, to understand that there's a difference between printed currency and most currency in circulation which is digital.  Most of the transactions you engage in every day, unless you're one of the few "cash only" people in the country, are done digitally either by card or check or wire transfer.  When you get a loan, you don't normally get a big bag of cash, but instead get a check or get a direct deposit to your account which you then spend using cards or checks.  
According the the Fed, "There was approximately $1.4 trillion in circulation as of February 18, 2016, of which $1.38 trillion was in Federal Reserve notes." (https://www.federalreserve.gov/faqs/currency_12773.htm)
But also according to the Fed,  there's $8.45 trillion in savings deposits in US institutions. (https://research.stlouisfed.org/fred2/series/WSAVNS) I don't know if that counts just savings or both savings and checking and investment accounts, but it's still significantly higher than $1.4 trillion.  
Then, the Fed reports $3.54 trillion in consumer debt, plus almost $13.8 trillion in mortgage debt, and $1.2 trillion in student debt which adds up to an even bigger difference from the $1.4 trillion than comparing it to savings deposits.  
Then we move to government debt.  The "national debt" is being reported at over $19 trillion and rising $1 trillion per year.  
But where does the government get it's money when it's accumulating this debt?  From the public.  And where did the public get its money?  From the banks as debt.  People will say "what about my paycheck", but trace that money back to where it started and the answer is "a bank as debt".
So the government is taxing and borrowing (via bond sales) to get money, which is debt, from the already circulating river of debt in the public hands (including foreign governments which buy the bonds too).
Through the BOND GAME, unlike in Monopoly where players GAIN when they pass go, the government LOSES (gains debt) every time the money passes through its hands just to be spent back into circulation to be borrowed yet again in a never ending process that exponentially grows the debt figure while never producing a net benefit to show for it. 
To SOLVE the debt, we must take these steps:
With a sovereign, debt free, private bank free currency, there is no need for government borrowing, no need for fractional reserve lending, and hardly a need, either real or perceived, for private banks. There also is no place in this debt free system for the nearly forced public usury we have in place now.